In March of 2011, an undersea earthquake sent tsunamis thundering across Japan, killing nearly 20,000 people and creating the most expensive natural disaster in history. Among the casualities was the Fukushima Daiichi Nuclear Power Plant, which was almost completely submerged by the tsunamis; an unprecedented event. Power was lost (obviously), cooling systems stopped, and the net result was a complete meltdown of three of the plant's reactor cores. It was a perfect storm of worst case scenarios. And now, even years afterward, some are calling it a worldwide radiation disaster, worse than even Chernobyl, that will produce a staggering death count for decades or even centuries. Today we're going to evaluate these assertions and see if we can separate fact from fiction.

The most important technical point to understand about various reactor kinds is the moderator. The moderator is a substance that slows down the fast neutrons being shed by the radioactive uranium fuel, converts the kinetic energy into thermal energy, and turns them into slow, thermal neutrons. A thermal neutron is much more likely to strike another uranium nucleus. This allows a chain reaction, in which the fuel produces enough heat to power a conventional steam generator. Most nuclear reactors use water as the moderator. Put uranium fuel rods into water, in the proper configuration, and you'll get a chain reaction.

Chernobyl, however, was a very different type of machine. It was what we call an atomic pile, the devices first designed during World War II to produce plutonium for atomic weapons. The atomic pile is literally a pile of graphite blocks, half a meter long and a quarter meter square, with a hole bored through the long axis. These graphite blocks were used as the moderator.

The problem with building a reactor out of graphite blocks is that graphite burns. Contain burning graphite within a concrete structure, and it explodes. This is exactly what happened at Chernobyl, and it's exactly why nobody would ever build a graphite-moderated reactor today; the whole reactor core was literally a bomb waiting to go off.

Three Mile Island and Fukushima were both water moderated reactors. This was one of the most significant safety improvements of the early 1950s. Fukushima's basic design is one of the earliest, called a BWR (boiling water reactor). The moderating water, which is also the cooling water, is directly boiled and drives a steam generator. The reason the Fukushima accident happened is that all sources of power were destroyed by the tsunami, including backups, backups, and their backups; and without the pumps to keep the system circulating, the cooling water boiled completely away, and the fuel melted. For months, firehoses sprayed water into the open reactors to prevent open flames from pumping radioactive smoke into the atmosphere. This contaminated water was barely containable; it leaked into the ocean, and was stored in anything that could be used as a tank.

Three Mile Island's design was a step newer, called a PWR (pressurized water reactor), and is of the same basic type as most existing plants. The main incremental safety feature here is that the core is kept pressurized to prevent the water from being able to boil; and as water's temperature goes up, its ability to moderate neutrons goes down. Thus the system is self-regulating: If the core gets too hot, it stops working efficiently, so it cools back down.

But like any system, this one was prone to breakdowns and human error. A broken valve allowed coolant water out of the core, and a confusingly designed instrument panel fooled operators into thinking the opposite was happening, who then let more out. By the time they figured it out, enough damage had been done that radioactive water had been able to mix with the separate water that runs through the steam generators. That radioactive steam was then released through the plant's filtration system, which removed nearly all the dangerous contaminants.

Fortunately, at Three Mile Island, only a tiny amount of radiation was released. It was an expensive cleanup and repair, but it was effectively contained; none of the containment structures were breached. There were no injuries, nobody within 16 kilometers received more than a chest x-ray's worth of radiation, and epidemiological studies predicted zero eventual deaths.

At the opposite end of the spectrum was Chernobyl. Since the plant openly exploded, a huge amount of very dangerous radioactive debris was spread over a large area. Two people were killed outright by the explosion, and a few dozen first responders were dead of acute radiation poisoning within three months. Estimates of eventual deaths vary wildly, with the most extreme coming from anti-nuclear activists; but the best epidemiological studies predict about 4,000 eventual cancer deaths in the region.

Reactors produce a lot of radioactive elements, but the two we care most about are iodine-131 (131I) and cesium-137 (137Cs). 131I is very dangerous, but fortunately it also has a very short half life of 8 days. After 10 half lives, or about 80 days, it's basically gone and no longer a threat. Three Mile Island produced about 560 GBq (gigabecquerels) of 131I; Chernobyl produced about 3 million times as much, about 1760 PBq (petabecquerels).

Where does Fukushima fit between those two? At the high end, about 500 PBq. That's about a million times more than Three Mile Island, and about a third of Chernobyl. But things were done very differently. The area around Fukushima was very quickly evacuated and prophylactic iodine was distributed just as fast. Neither of these things were done at Chernobyl in a timely manner. The result was that in those first few weeks when the 131I presented a risk, lots of people near Chernobyl were exposed, and almost nobody was exposed in Fukushima.

Then there's the 137Cs, which is the long term threat about which bloggers and reporters expressed so much fear in the years following Fukushima. No significant amount was released at Three Mile Island because it was filtered out; but at Chernobyl, 85 PBq were scattered over the surrounding area. It has a half life of 30 years, which is a really long time; and it's why we're probably going to have to wait for a few more 30-year half lives to go by before the area around Chernobyl will be safe to move back in. At Fukushima, 137Cs was in the smoke from burning fuel and was in the cooling water sprayed into the damaged cores. A maximum of 15 PBq, just over a sixth as much as Chernobyl, was released into the environment. How much 137Cs does it take to produce 15 PBq of radiation?

About 4.7 kilograms. That's a lump a little smaller than a tennis ball.

1 gram of 137Cs = 3.214 TBq (terabecquerels of radiation) A becquerel is the equivalent radiation of one neutron decaying per second.

Now, don't think that sounds like an absurdly small amount. 4.7 kilograms is a lot of atoms; if spread into an atomically thin pancake it could easily cover the entire world.

As with all dangerous radioactive elements, we've done a lot of study on the health effects of 137Cs. Although it has a radioactive half life of 30 years, it has a biological half life of 70 days. 70 days is the time it takes for animals like us to get half of it out of our system. That can be accelerated to just 30 days with treatment. From animal testing done in the 1970s, we know that 140 MBq per kilogram of body weight is a lethal dose. So with a little algebra, we know that Fukushima's 4.7 kg was enough 137Cs was enough to kill one and a half million people (assuming 80 kg people).

So here's the big "BUT..." — almost none of the will ever find its way into anyone's body. The Fukushima contamination is detectable all over the world, and it's probably in the bodies of everyone living on the planet. That's the nature of entropy. There is a lot of it in the water stored in tanks at the Daiichi power plant and in the soil in the evacuated zone, but the rest of it has been carried by water and atmospheric currents everywhere.

Our oceans contain one and a half billion cubic kilometers of water. Dilute Fukushima's 4.7 kilograms into that, and every cubic kilometer of water would contain less than one thousandth of a lethal dose. In other words, to die from Fukushima's radiation, you will need to drink one thousand cubic kilometers of seawater, and somehow manage to absorb every atom of 137Cs from it. But if you're looking only to eventually get cancer, then you might be able to do so on only a few hundred cubic kilometers of the Pacific.

But if you tried to do that, you would already die one million times over from just the primordial radioactive elements that exist naturally in our oceans; more than 15 zettabecquerels (a million petabecquerels) of naturally occurring potassium-40, rubidium-87, uranium-238, and so on.

This is the central thesis of science reporters who have been desperately trying to respond to scientifically illiterate fearmongerers printing headlines like "Your Days of Eating Pacific Ocean Fish Are Over" and "28 Signs That The West Coast Is Being Absolutely Fried With Nuclear Radiation From Fukushima". Our planet's entropy has, long ago, already rid itself of any credible threat from the Fukushima radiation, outside of the immediate evacuation zone. Fishing has long been suspended from Daiichi's vicinity, so there is no way that eating a legally caught fish can give you any significant Fukushima radiation.

The Fukushima disaster will probably end up being the most expensive industrial accident and cleanup in history, but it has certainly not been among the most dangerous, thanks largely to Japan's prompt action. The newest World Health Organization assessment concludes:

...No discernible increase in health risks from the Fukushima event is expected outside Japan. With respect to Japan, this assessment estimates that the lifetime risk for some cancers may be somewhat elevated above baseline rates in certain age and sex groups that were in the areas most affected.

Clearly it wasn't good, but if you want to be able to develop proper response plans, you have to understand the correct facts about the situation. Absurdly exaggerated and sensationalized reports do not help anyone; rather they increase confusion, and decrease our ability to respond to such events appropriately.

Correction: An earlier version of this incorrectly stated that the control rods were not inserted at Fukushima due to the power failure following the tsunami. They were, in fact, automatically inserted after the earthquake, before the tsunami struck.